beldam (n.)

also beldame, "aged woman," 1570s; earlier "grandmother" (mid-15c.), from dame (q.v.) in the sense of "mother" + bel-, Middle English prefix expressing relationship (as in belfader, belsire "grandfather"), from Old French bel, belle "beautiful, fair, fine" (see belle). This "direct relationship" sense of bel is not found in French, where the prefix, however, is used to form words for in-laws.